
@testset "simplify" begin
    
    @test simplify(:(10 - 5)) == 5
    @test simplify(:(u - 2)) == :(u - 2)
    @test simplify(:(1 * (u - 2))) == :(u - 2)
    @test simplify(:(1.0 * cos(x1))) == :(cos(x1))
    @test simplify(:(2 * 3x)) == :(6x)

end
